Bipolar junction transistors (BJT) and heterojunction bipolar transistor (HBT) integrated circuits (ICs) have developed into an important technology for a variety of applications, particularly as power amplifiers for wireless handsets, microwave instrumentation, and high speed ( greater than 10 Gbit/s) circuits for fiber optic communication systems. Future needs are expected to require devices with lower voltage operation, higher frequency performance, higher power added efficiency, and lower cost production.
Unlike BJTs in which the emitter, base and collector are fabricated from one semiconductor material, HBTs are fabricated from two or three dissimilar semiconductor materials in which the emitter semiconductor material has a wider band gap than the semiconductor material from which the base is fabricated. This results a superior injection efficiency of carriers from the base to collector over BJTs because there is a built-in barrier impeding carrier injection from the base back to the emitter. Selecting a base with a smaller band gap decreases the turn-on voltage because an increase in the injection efficiency of carriers from the base into the collector increases the collector current density at a given base-emitter voltage. However, in order to further increase the speed of the devices and further increase the collector current density at a given base-emitter voltage it would be desirable to minimize the transit time of carriers across the base into the collector.
InP-based heterojunction bipolar transistors (HBTs) are being developed for 40 Gbps lightwave circuits and wireless applications. Compared to GaAs-based HBTs, InP/InGaAs HBTs have lower turn-on voltage and a higher frequency of operation. The dominant growth technology in InP production has been molecular beam epitaxy (MBE), which typically employs either beryllium or carbon doping in the InGaAs base. Carbon-doped InP/InGaAs HBTs have demonstrated improved reliability in comparison with beryllium because carbon has a significantly lower diffusion rate in InGaAs. However, because MBE is not a multiwafer growth technique, it is impractical for large scale manufacture of wafers. Metalorganic chemical vapor deposition (MOCVD) can be used for multiwafer growth and therefore, is more suitable for large scale manufacture of HBTs. However, carbon-dopant levels in the base layer of HBTs fabricated using MOCVD have been relatively low. Thus, a more economical method of preparing InP-based HBT having a relatively high carbon dopant level in the base layer is necessary in order for InP-based HBTs to become reliable, cost-effective commercial circuits.
The present invention provides HBTs, and methods of fabricating HBTs, having a carbon-doped base layer composed of gallium, indium, and arsenic fabricated using MOCVD epitaxial growth system. The method involves growing a carbon-doped base layer represented by the formula InxGa1xe2x88x92xAs in which x is less than 1. In a preferred embodiment, x is about 0.4 to about 0.6. The base layer is grown from a gallium, indium, and arsenic source over an n-doped collector layer. Preferably, the collector is composed of InP or Inxxe2x80x2Ga1xe2x88x92xxe2x80x2As in which xxe2x80x2 is less than 1. An n-doped emitter layer is then grown over the base layer. Preferably, the emitter is composed of InP grown from an indium and a phosphorous source or InyAl1xe2x88x92yAs in which y is less than 1 grown from an indium, aluminum and arsenic source.
In one embodiment, the base layer is doped with carbon, preferably at a concentration of about 1.5xc3x971019 cmxe2x88x923 to about 5.0xc3x971019 cmxe2x88x923. When the base layer is doped with carbon, the carbon is preferably incorporated in the base layer by growing the base layer in the presence of an external carbon source such as carbon tetrabromide or carbon tetrachloride.
In another embodiment, the composition of the base layer is graded so that the value of x is large at the surface of the base nearer to the collector than the surface of the base nearer the emitter. Preferably, the value of x is linearly graded across the base layer.
The method of the invention which utilizes MOCVD to fabricate InP-based HBTs is more cost effective than other methods of manufacturing InP-based HBTs. In addition, a base layer having a relatively high concentration of carbon dopant can be prepared using an external carbon source in the method of the invention. A high dopant concentration in the base layer is desirable because it decreases the base sheet resistivity (Rsb) of the base resulting in increased electron mobility across the base and increased collector current gain at a given emitter/base voltage. Thus, the development of carbon-doped InP/InGaAs HBTs grown by MOCVD will accelerate the insertion of InP HBTs into reliable, commercial circuits in a cost-effective manner.
Grading the composition of the base layers as disclosed in the method of the invention results in a graded band gap across the base layer in which the band gap is larger at the base-emitter interface and becomes gradually smaller at the base-collector interface. This introduces a quasielectric field which accelerates electrons across the base and thereby improving the speed of the device. In addition, increased transit of electrons across the base reduces hole-electron annihilation in the base and thus increases the current gain.